1. Field of the Invention
The present invention relates to a fuel cell generation system and more particularly to an alarming and protecting system for the cooling water supply provided to the fuel cell.
2. Description of the Prior Art
Recently, attention has been paid on a fuel cell generation system for directly converting fuel energy into electric energy. This fuel cell generation system normally comprises a fuel cell consisting of a pair of porous electrodes and an electrolyte material located between these electrodes, a fuel such as hydrogen coming into contact with the outer side of one electrode and an oxidizer such as oxygen coming into contact with the outside of the other electrode so that the system serves to generate electric energy between the electrodes, which energy results from the electro-chemical reaction caused by the contact of these gases. It can generate electric energy at high conversion efficiency as long as the fuel and oxidizer are supplied through the system.
FIG. 3 is a vertical section showing arrangement of a normal fuel cell and FIG. 4 is a plan view showing the same arrangement. As shown, the normal fuel cell comprises a cell stack 1 having a plurality of cells, the single cell consisting of an electrolyte layer formed by impregnating a matrix with phosphoric acid served as electrolyte and a pair of porous electrodes on the both sides of the electrolyte layer. The cell stack 1 includes manifolds 2, 3 used for supply and discharge of a reaction gas.
The manifolds 2 are connected to a fuel supply pipe 4 and a fuel discharge pipe 5 so that these pipes 4, 5 are located on both opposite sides of the cell stack 1. It is also connected to an air supply pipe 6 and an air discharge pipe 7 so that these pipes 6, 7 are perpendicular to the pipes 4, 5 and are located on both opposite sides of the cell stack 1.
And, the cell stack 1 provides the top pressure plate 9 and the lower pressure plate 10 on the upper and the lower surfaces through insulating plates 8 located therebetween. These pressure plates have fastening members consisting of rods 11 and nuts 12 fastened at each end of the upper and the lower surfaces of the cell stack. The fastening members serve to firmly fasten a plurality of cells together.
And, the fuel cell main body described above is fixed on a support 15 with bolts. The support 15 is located above a lower tank 14. Parts of tanks, 16 and 17 cover the whole of the fuel cell assembly. Further, a plurality of cooling plates 18 are inserted into the cell stack 1 for removing heat generated within the fuel cell.
The fuel cell stack 1 is cooled to a proper operating temperature by cooling water which flows through passages formed in the cooling plates. The cooling water containing excessive heat is discharged out from outlet header 20.
It is well known that such a cooling system for a fuel cell has two types, that is, a pressurized water cooling system and a boiling water cooling system.
The pressurized water cooling system requires far more cooling water than the boiling water cooling system, resulting in disadvantageously greater auxiliary power and thereby lowering efficiency of the fuel cell generation system. Hence, the fuel cell normally employs the boiling water cooling system. This system uses latent heat of vaporization contained in the cooling water, thereby reducing the amount of the cooling water and the auxiliary power.
The boiling water cooling system is designed to give heat to the cooling water supplied from an inlet header 19. As the water flows through the cooling plates 18 it starts and keeps boiling and is discharged as a two-phase flow consisting of a liquid phase and a vapor phase.
And, the two-phase flow flows through cooling plate outlet pipes 23 and are all gathered at the outlet header 20. Then, the gathered two-phase flow is discharged through the penetration part 21. Finally, it comes into a steam separator 22 through a water outlet pipe 24 of a cell stack 1.
In this case, the fuel cell main body employing the boiling water cooling system has each pressure of the cooling water applied on each cooling plate, that is, cannot offer uniform boiling, because the cooling plates stacked vertically in the cell stack 1 have respective heights so that the cooling water has respective water heads at respective cooling plates as it flows through the cooling plates. Hence, the quality of the two-phase flow is different at the cooling plate outlets extending from the upper to the lower parts of the cell stack. In general, therefore, the amount of cooling water flowing through the upper part of the stack is smaller than that flow through the lower part of the stack. This tendency distinctly occurs in a case where the amount of the cooling water flowing in the fuel cell is reduced by some reason or the load applied on the fuel cell is made excessive. At worst, in the cooling plate located at the upper portion of the cell stack, the cooling water is substantially boiled away, disadvantageously resulting in extremely lowering cooling efficiency of the cooling plates and allowing cell temperature to reach its established upper limit value or more.
However, no acceptable means has been proposed for properly detecting in abnormal state of a cooling water supply system extended over the vertical length of the cell stack. At the present state, hence, the fuel cell may be kept operating though it comes into an abnormal state, resulting in breaking down the fuel cell itself.
For monitoring the abnormal state, for example, by inserting a temperature sensor like a thermocouple into the upper cell, it is possible to detect an abnormal temperature of the cell. The fuel cell used in a normal generation system, however, has so high a voltage applied on the cells that the temperature sensor cannot be sufficiently insulated. It is thus difficult to precisely measure the cell temperature. In addition, the corrosive environment within a fuel cell stack makes temperature measurement very unreliable.